Chromatography generally involves a separation process for redistributing molecules in a mixture from a thin material phase to a second bulk material phase. The two phases in contact may be liquid-liquid, liquid-solid, vapor-solid or vapor-liquid. Where the bulk phase is a gas stream, the term gas chromatography is used to describe the process. The gas stream is then analyzed to obtain information specific to the transfer molecules. Typical detection systems include thermal conductivity, flame ionization and argon detectors. More recent detectors include electrolytic conductivity detectors providing an electrical signal which is functionally related to the presence of a selected element.
Electrolytic conductivity detectors have long been available for investigating the properties of electrolytes in solutions. Such devices in the prior art typically include electrode surfaces with a continuous phase liquid electrolyte therebetween. Conventional use of electrolytic conductivity detectors in a gas chromatograph system generally follows this prior art approach and separates any gas phase from the liquid phase prior to the detector cell which provides an output related to the electrolyte.
An early typical electrolytic conductivity detector, the Coulson detector, is depicted in the Journal of Gas Chromatography, April 1965, at pages 134-137. As therein depicted the electrolytic conductivity detector cell provides a liquid collecting and gas venting section for separating liquid from the gas flow and thereafter introducing the liquid to an electrode volume where the liquid electrolyte is used to form a continuous conductive path between electrode structures in the sidewalls. Yet another electrolytic conductivity detector is depicted in U.S. Pat. No. 3,934,193 to Hall. The Hall detector also separates the flow into a gas flow and a liquid flow and the liquid flow is captured between two concentric electrode surfaces for conductivity measurement. Thus, in both detectors, a slow moving volume of liquid is formed having a relatively long dwell time with respect to chromatographic events.
The Coulson conductivity cell is typically a complex glass device which is difficult to interface with many chromatographs. Further, gases can accumulate in the detector section between electrodes. These gas bubbles can be difficult to displace and the liquid phase tends to merely divert about the gas phase bubble rather than to sweep away the bubble.
The Hall conductivity detector is a substantially more rugged detector and easier to interface than the Coulson detector but the separating chamber is subject to deterioration. The relatively large electrode area per fluid volume ratio produces good sensitivity to the conductivity of the electrolytic fluid but can require frequent cleaning to maintain the volume cleanliness. Thus, it would be desirable to provide an electrolytic conductivity detector which is not subject to blockage by gas accumulation and which is substantially self cleaning, while retaining the large signal-to-noise ratio which is desired for sensitive results.
It will be appreciated that the signal-to-noise ratio can be affected by short term variations in the fluid flow. Such short term variations are typically induced by the pump which moves the solvent liquid, used to dissolve portions of the gas phase, through the system and through the electrolytic conductivity detector. The signal-to-noise ratio could desirably be improved by reducing or eliminating the induced flow fluctations.
It will also be appreciated that gases evolving directly from the sample being measured are reacted with an oxidant or a reductant to form reaction products suitable for use in combination with an electrolytic conductivity detector. Thus, a reactor is typically furnished having a quartz or nickel reaction tube within a heated volume. Access to the reaction tube is desirable to check for reaction tube cleanliness and tube integrity. However, prior art reactor assemblies generally require removal of the reactor from the analysis system in order to change the reaction tubes. Further, the integrity of the reaction tube is often jeopardized by the system used for supporting the reaction tube within the reactor chamber.
The disadvantages of the prior art are overcome by the present invention, however, and improved apparatus is provided for electrically detecting gas chromatograph products and for enhancing the overall sensitivity of a gas chromatograph with an electrolytic conductivity detection system.